1. Field of the Invention
The present invention relates to a pulsed load cooling method and refrigerator.
The invention more particularly relates to a pulsed load method for cooling a component of a “Tokamak”, i.e. a component of an installation for intermittently generating a plasma, the method employing a cooling device that subjects a working fluid such as helium to a working cycle comprising: compression; cooling and expansion; heat exchange with the component; and heating, the Tokamak comprising at least one operating mode called a “periodic and symmetric” operating mode, i.e. an operating mode in which plasmas of preset duration Dp are generated periodically with intervals of duration Dnp between two successive plasmas, the duration of the intervals being at most 30% different to the duration Dp of the plasmas (Dnp=Dp±30%), according to the method the cooling power produced by the cooling device is increased to a relatively high level when the Tokamak is in a plasma generation phase whereas the cooling power produced by the cooling device is reduced to a relatively low level when the Tokamak is no longer in a plasma generation phase.
The invention more particularly relates to a pulsed load cooling method and refrigerator for cooling a component of a Tokamak, i.e. a component of an installation for intermittently generating a plasma.
2. Related Art
A Tokamak (Russian acronym for “Toroidalnaya Kamera c Magnitnymi Katushkami”) is an installation capable of producing the physical conditions necessary for obtaining power from fusion. In particular, a Tokamak intermittently produces plasma, i.e. ionized gas that conducts electricity.
The cooling requirements of Tokamaks depend on their highly transitory operating state. A Tokamak produces plasma in discontinuous, repeated bursts. The plasmas are generated cyclically at regular intervals or else randomly, on request.
This operating mode requires what is called pulsed load cooling, i.e. very substantial cooling power is required for a very short amount of time (during the plasma generation phase), this high cooling demand being followed by a longer period (until the next plasma is generated) during which there is little need for cooling.
Tokamak refrigerators are therefore designed to meet the requirements of this operating mode. Thus, these refrigerators employ what is called an “economizer” mode, producing liquid helium in the periods between plasmas. The liquid helium produced is stored in a reserve that will be consumed by boiling to cool a component of the Tokamak during plasma generation phases.
When the period between two plasmas is sufficiently long, the maximum filling level of the liquid helium reserve is reached before the following plasma. The cooling power of the refrigerator may then be reduced, thereby saving a substantial amount of power. In a conventional solution, the power of the refrigerator is minimized by reducing the pressure of the cycle (i.e. by reducing the pressure level of the compression of the helium in its working cycle). The power of the refrigerator may also be decreased or increased by changing the cycle flow rate when a frequency variator is used (i.e. the flow rate of helium through the working cycle is selectively decreased or increased).
A heater is generally provided in the liquid helium reserve. This heater is activated in order to consume excess cooling power, so as to keep the liquid level constant or at least below a maximum threshold.
Conventionally, when a new plasma is generated, the refrigerator is made to produce a maximum cooling power either manually (by an operator), or as a function of the “heating curve” of the heater.
When the plasma has been extinguished and the cooling power required is lower, the return of the refrigerator to a regime producing less cooling power is generally achieved automatically, when no power is being supplied to the heater.
Although this operating mode is satisfactory overall, the power consumption of the refrigerator remains high.
Certain Tokamaks operate by repeatedly, periodically and cyclically generating plasmas the periodic profile of which approximates to a sinusoidal mode, i.e. the plasma phases and the plasma-less phases follow one another periodically with identical or substantially identical durations.
Although the plasma is generated in on/off mode, the thermal load (i.e. the cooling requirement) seen by the refrigerator resembles a sinusoidal wave.
Conventionally, the power of a refrigerator is regulated by matching the pressures and flow rates of the working cycle of the working fluid (helium for example) to the cooling requirements.
For the sake of simplicity, this working fluid is called “helium” in the rest of the description. Of course, this working fluid is not limited to this gas alone but may comprise any appropriate gas or gas mixture.
The working cycle of a high-cooling-power refrigerator very often comprises three cycle pressure levels: a high pressure (HP), an intermediate pressure (MP) and a low pressure (BP). Sometimes, when required, the helium is subjected to other additional pressure levels. For example, the working cycle may comprise a stage in which the helium is subjected to a subatmospheric pressure (LP).
The method and the device according to the invention are however not limited to a particular number of pressure stages.
The high-pressure helium HP is fed to systems in which a substantial degree of expansion occurs, such as for example Brayton turbines, cold turbines or Joule-Thomson valves.
The intermediate pressure MP helium is generally needed to limit the power of the Brayton turbines, which are not always technically capable of completely expanding the helium between the high pressure HP and low pressure BP levels.
There may then be a limited expansion between the high pressure HP and intermediate pressure MP levels. The presence of an intermediate pressure MP level may moreover have the benefit of increasing the efficiency of the compressors, by staging the helium compression.
Optimized pressure stages are widely designed using the theoretical value MP=√{square root over (BP×HP)}. The low pressure BP corresponds for its part to the saturation pressure of the helium and to the inlet pressure of the compressors. When the required helium temperature is below 4.3 K, the saturation pressure is subatmospheric (LP), and an additional compression stage is then necessary between the subatmospheric pressure LP and the low pressure BP.
The inventors have observed that regulating the power of the refrigerator so as to properly meet the cooling requirements of the Tokamak is difficult and requires a lot of effort on the part of the operators. In particular, controlling for the regularity of the thermal load of the Tokamak is relatively difficult. If the power of the refrigerator is poorly regulated, the inventors have observed that the liquid helium level in the buffer may constantly be increasing or decreasing from one cycle to another. As a result it is necessary to interrupt the sequence of plasma generation in order to allow the liquid helium level in the buffer tank to return to its initial state, or to a given fill level, in an attempt to keep this level between the maximum and minimum thresholds for as long as possible—this being necessary for safe operation of the installation.
The article “Using Dynamic Simulation to support Helium Refrigerator Process Engineering” Proceedings of ICEC 22-ICMC 2008, pages 39-44, by Dauguet P.; Briend P.; Deschildre C. and Sequeira S. E., describes a cooling method in which the cooling device is controlled so as to keep the working cycle pressures constant and also so as to keep the electrical power consumed by the cooling device constant.